Currently

• check.py will download the wasm waterfall build of llvm+clang and use that to test the wasm backend. This works on travis and locally.
• check.py will look for emcc in the path, and it that is present, it will use that for testing, which tests fastcomp and asm2wasm. This works locally, but not on travis since we don't have emcc set up there.

It would be nice to use the emsdk to do the tests in that second point on travis. There is now a nightly build of the emsdk that @juj set up,

• https://s3.amazonaws.com/mozilla-games/emscripten/packages/llvm/nightly/linux_64bit/emscripten-llvm-latest.tar.gz
• https://s3.amazonaws.com/mozilla-games/emscripten/packages/emscripten/nightly/linux/emscripten-latest.tar.gz